Fun with Neutral Pressure Planes (NPP)

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Permalink Reply by Tom Mallard on September 5, 2012 at 2:57pm

Don't you also need to know elevation to know D-P since height within a structure is used? ... once it's opened to the outside the elevation matters to density over that height at any D-T.

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Permalink Reply by Robert Riversong on September 5, 2012 at 3:06pm

I'm not sure which post you're responding to or exactly what your question is.

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Permalink Reply by Bob Blanchette on April 7, 2012 at 12:56pm

About the only atmospherically vented appliance left in most homes are tank water heaters. Most furnaces, tankless water heaters, and dryers use induced draft blowers.

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Permalink Reply by Ed Ballard on April 7, 2012 at 2:17pm

Bob-------In my area your comment could not be further from what I see everyday. 80% and less are way more common than we would like. Condos on slabs are culprits. The furnace is either in the attic space or a closet. Other than the cheaper route by the builders you have the problem of routing the condensate drain through un-heated areas ( freezing ) In older housing stock built on slabs you have the same freezing problems as the CAZ is generally in the center of the home and the only route for the drain is up and out. The DWH is in a closet area with hi/low combustion air coming in from the attic. I see older homes everyday w/ furnace in basement that need replaced but the HO is not about to fork out until it fails. People don't generally plan on furnace upgrades until they fail. I wish your comment applied in my area ( OHIO )

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Permalink Reply by Bob Blanchette on April 7, 2012 at 2:40pm

We still see a few old furnaces, but they aren't common in Oklahoma. Most people replace the furnace when their A/C goes out. Sometimes we do see a few furnaces from the 80's where the A/C has been replaced multiple times. Moving a furnace is no small undertaking, they are rarely relocated when they are replaced.

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Permalink Reply by Patrick Michaelyan on April 10, 2012 at 3:45am

Hello Bob and Ed.

I too only wish the situation that you have witnessed Bob was taking place in my area. I have now performed 30 audits in the last 3-4 months, and of these 5 folks had new furnaces\boilers installed. Each one is an atmospherically-vented appliance, and the homeowners all are upper-middle income. I did have one small win to get excited about, when a homeowner decided to go with a high-efficiency unit after I spent considerable time convincing them.

Ladies and gentlemen, I am so glad to see so much constructive conversation on NPPs and stack effect. At the same time, I wonder what this means when so many homes are like sives, with rapidly aging combustion appliances in them. And, remember, I mainly serve upper-middle income homeowners. My days in low-income really can get one down.

Patrick

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Permalink Reply by Bud Poll on April 10, 2012 at 7:05am

Patric, Ed, my neighborhood is much like what you are describing and Patric, it does get depressing.  My market is primarily just above the low-income and a pellet stove addition would be far more likely than a new high efficiency.  

One of my concerns about many of these old leaky homes is that much of the easy savings involve air sealing.  I've seen chimney chases (plural) big enough to climb from basement to attic.  Easy fix, but what will that huge change in leakage do to everything else.  Add in a case of great stuff in the basement and attic and suddenly the old pot belly furnace in the basement is searching for combustion air.  Throw in a gas water heater and an old fireplace and these home owners need a serious education, before they start their DIY improvements.  And we are the ones that should know how to explain the "why" and stack effect becomes part of that discussion.

Bud

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Permalink Reply by John Brooks on April 7, 2012 at 4:41pm

I am including some example outside pressures in this drawing.

Bud Poll's worksheet is based on an outside atmospheric pressure of 101,325 Pascals

(a common value for sea level)

I have decided to imagine that my "house" is at a slightly higher altitude.

I chose a round number... 100,000 Pascals

This example shows Two open windows.

I am thinking that the NPP would be about 10 feet above the floor.

(Approx midway between the two windows.)

The pressure WRTO would be + 5.5 pa at the top and -2.5 pa at the bottom.

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Permalink Reply by John Brooks on April 8, 2012 at 4:24am

again this a a theoretically tight house

 

I think Bud Poll knows the answer for this...

but first.....Anyone else?

Where is the Neutral Pressure Plane for this example?

(how high above the floor)

 

What are the pressures at the floor and the peak WRTO (with respect to the outside)?

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Permalink Reply by John Brooks on April 8, 2012 at 7:08am

I forgot to add something to my hypothetical question.....

Assume the the "House" will be heated and maintained at 70 degrees

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Permalink Reply by Robert Riversong on September 5, 2012 at 12:26pm

John, I just found this thread. Good stuff!

I would say that this is identical to a single opening through the wall at 6', which would put the NPP at the vertical center of that opening, which in this case is right at the bottom of the flue.

But, because it's a vertical stack that terminates in a lower pressure zone, the CFM would be much greater than a similarly-sized horizontal opening (if the flue were 6" diameter, the flow rate would be 90 cfm) and so it would make the house below the flue entry strongly negative and mildly positive above it.

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Permalink Reply by Robert Riversong on September 5, 2012 at 2:02pm

Correction: In doing some research to respond to other issues here, I stumbled across the answer to this puzzle and my first guess was incorrect. 

The open flue would serve as a passive exhaust "fan" and hence lower overall house pressure, reducing natural exfiltration (the positive pressure zone) and increasing infiltration (the negative pressure zone), thereby raising the NPP (see figure d).

These illustrations are from a wonderful archived article at the National Research Center of Canada:

CBD-245. Mechanical Ventilation and Air Pressure in Houses

http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-245.html

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